1. Field of the Invention
The present invention relates to a system for NOX reduction. Particularly, the present invention is directed to a system for converting urea into reactants for removing NOX from industrial emissions.
2. Description of Related Art
A variety of urea conversion devices are known in the art for converting urea into reactants, such as ammonia, which are useful in reducing NOX emissions in industrial settings. Of such devices, many are directed to systems that utilize hydrolysis to convert urea into ammonia and other reactants for NOX reduction.
Combustion of fossil fuels, such as in power plants and other industrial settings, leads to a release of pollutants. NO2 and NO (referred to as NOX) are particularly problematic pollutants arising from fossil fuel combustion. Great efforts have been applied to the reduction of NOX emissions. Selective Catalytic Reduction (SCR) is one process that has achieved relative success in NOX reduction. SCR reacts ammonia or other reactants with NOX in effluent gasses to reduce NOX into more environmentally friendly products. It is possible to reduce in excess of 90% of the NOX out of effluent gasses through SCR. Another variant of SCR is Selective Non-catalytic Reduction (SNCR), which can similarly use ammonia to reduce NOX, albeit at a higher temperature.
The ammonia typically used in SCR and SNCR presents problems of its own, however. The most economical form of ammonia for use in SCR and SNCR is anhydrous ammonia, but classification of this reactant as a hazardous chemical may restrict its use in some locations. Aqueous ammonia is commonly used to avoid the hazardous chemical classification. But the costs of transportation, storage, and processing of aqueous ammonia are great, especially considering the fact that most of what is shipped, stored, and processed is the water, which can be in excess of about 70% by volume. This cost may restrict the use of aqueous ammonia.
In order to avoid the costs and hazards of transporting and storing anhydrous and aqueous ammonia, on-site production of ammonia is commonly used in conjunction with SCR and SNCR. Ammonia suitable for SCR and SNCR can be produced from urea, which is not hazardous and can be inexpensively transported in its solid form. Typically, a hydrolysis process within a saturated steam-water vessel is used to produce gaseous ammonia and other useful reactants from solid urea. It is also possible to generate ammonia and other useful reactants from urea by gasifying urea in a stream of combustion gases to decompose the urea into useful reactants, as described in U.S. Pat. No. 7,090,810 to Sun et al.
U.S. Pat. No. 6,730,280 to Cooper et al. describes a method for producing ammonia from solid urea. Solid urea is mixed with water into an aqueous solution. The aqueous urea is then processed in a pressurized reactor in which heat is applied to promote hydrolysis of the urea. Gaseous ammonia, carbon dioxide, and steam bubble out of the liquid in the bottom of the reactor. These gasses accumulate at the top of the reactor, and can then be introduced into flue gasses to reduce NOX emissions therefrom.
U.S. Pat. No. 5,252,308 to Young describes a method for producing ammonia from urea using an acid. An aqueous solution of urea is introduced into a reactor, which includes a vessel containing concentrated liquid phosphoric acid. Ammonia and carbon dioxide are liberated in a gaseous form within the reactor, and can then be introduced into flue gasses for NOX reduction.
U.S. Pat. No. 7,008,603 to Brooks et al. describes a process for converting urea to ammonia in an on demand basis. A control system is implemented to control the temperature and pressure of a pressurized reactor in such a manner as to release a desired amount of ammonia. Urea can be supplied to the reactor from solid urea mixed into an aqueous solution, or as molten urea. Heating coils can supply the needed heat to the liquid reactants in the reactor.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. However, the state of the art urea hydrolysis reactors have large pressure vessels holding standing liquid. Thus they take up valuable space and controlling their reaction rates can be difficult. Typical hydrolysis reactors are heavy and operate at high pressures, which raises safety concerns. Known hydrolysis reactors have significant reactant volumes, which can lead to complications during start up and shut down. Moreover, the bubbling of ammonia and other gases out of the liquid state of the known urea hydrolysis reactors can cause a foam layer to build up. This, along with the build up of additives commonly used in solid urea, can lead to an accumulation of contaminants within the reactor, requiring frequent down time for cleaning and maintenance of the reactor. Although solutions to some of these problem have been developed, such as the method for removing contaminants in reactors described in U.S. Pat. No. 6,511,644 to MacArthur et al., there still remains a continued need in the art for low maintenance reactor for producing ammonia from urea. There also remains a need in the art for a urea conversion reactor that is inexpensive and easy to make and use. The present invention provides a solution for these problems.